Look Into the Future

The immediate future for InSAR looks good with the work of the ERS satellites being continued by ESA's Envisat, launched in 2002, and launches of the Japanese ALOS satellite and Canadian Radarsat-2 planned this year. These satellites will extend the lifespan of this new technology, but none of them will greatly improve upon current capabilities. That requires something new: A dedicated InSAR mission, targeted at earthquake and volcanic hazards. Such a mission would have onboard GPS to control and measure the satellite's orbit to a very high precision; it would operate at L-band (20 cm) wavelength to ensure better coherence in vegetated areas; it would collect data on every satellite pass, enabling detailed time series to be created, atmospheric noise to be reduced, and faults with low slip rates to be monitored/identified; it would acquire data from several look directions allowing 3D displacements to be recovered; it would provide data cheaply and quickly to the scientific community. Proposed missions with these specifications are being considered by NASA and ESA, and there are reasonable prospects that one will be launched within the next 5-10 years.

Many of the last decade's significant earthquakes occurred on faults that had not previously been recognised as major faults of their regions [e.g. Northridge, CA (1994); Kobe, Japan (1995); Athens, Greece (1999); Bam, Iran (2003)]. It is here that InSAR could have a big impact, by mapping strain accumulation globally and producing reliable medium-range earthquake forecasts. The human argument for such a mission is compelling: A tenth of the world's population lives in areas classified as having medium to high seismic hazard by the Global Seismic Hazard Assessment Program. Earthquake fatalities are highest in developing countries, which cannot afford ground-based monitoring equipment.

The economic argument is also simple. The 1994 Northridge earthquake in Los Angeles caused total property damage estimated at $20 billion. This would have been greater were it not for an intense program of hazard mitigation activities over the previous two decades. Rebuilding or retrofitting structures to protect them from earthquakes is relatively cheap compared to the cost of rebuilding after an earthquake. For example, the US Federal Emergency Management Agency estimate the cost of retrofitting bridges to be just 22% of the cost if they are destroyed by earthquakes, and this does not take into account the cost to the local economy of the temporary loss of infrastructure. Although the cost of a dedicated InSAR mission is high (ca. 150-250million), a city saved from extensive earthquake damage after an InSAR forecast led to a major retrofitting program might consider the price tag cheap. A satellite-based system is also much cheaper than attempting to make similar measurements using ground based techniques such as continuous GPS.e

A dedicated InSAR mission in the next ten years, and perhaps a constellation of Earth monitoring satellites in the next 25 years, will lead to a vastly improved understanding of the physics of the earthquake cycle, a complete time-varying map of the Earth's strain and reliable earthquake forecasts. Ultimately, this will save lives.

eCovering just the populated areas of the planet at risk from earthquakes with continuous GPS instruments spaced on a 15 km grid would cost ca. 1 billion.

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